The present invention relates to a method and a system for measuring the quality of digital television signals, and it is more particularly intended to managing the quality of service of a network for distributing digital audiovisual signals.
In order to be able to supervise a distribution network effectively, it is necessary to make use of quality measurements performed on various signals in the network, said signals serving to transport and broadcast audiovisual signals.
An object of the present invention is to provide a system making such supervision possible, without giving rise to additional complication, and without having to take account of characteristics that are specific to digital systems, for example the characteristic of the MPEG-2 system whereby the images of a sequence are not transmitted in the same order as they will be seen on a screen (the images I are transmitted, and then the images P, and finally the images B from a group of images GOP).
The invention is based on the idea of using a reference time signal that is already present in the data stream to label measurements.
The invention thus provides a method of measuring the quality of digital television signals comprising a data stream, said method including measuring quality and producing measurement signals representative of the quality of at least some of the data of the data stream, and being characterized in that it comprises the following steps:
a) selecting a reference time signal from the data stream;
b) generating a multibit digital clock signal on the basis of the reference time signal, with the phase of the clock signal being locked on the basis of said reference time signal, and with the count thereof being deduced unambiguously from the count of a time base generated during initial encoding of the digital television signals; and
c) labelling said measurement signals using at least some of the bits of the digital clock signal, that are meaningful given the nature of the measurements performed.
Advantageously, step a) comprises the following sub-steps:
a1) selecting digital frames corresponding to video data associated with a program from said data stream; and
a2) selecting said reference time signal from said frames.
By way of example, the digital television signals are in the MPEG-2 standard and, during step a1), frames are selected on the basis of the PID field.
Preferably, the video data is the data on which audio and/or video quality measurements are performed. In which case, the method may include a step of decoding video data from frames selected on the basis of said PID field.
The selection of the reference time signal may be performed by testing a flag indicating the presence of said reference time signal in each frame under consideration.
For digital television signals in the MPEG-2 standard, the reference time signal can be the signal PCR, the signal SCR, the signal DTS, or indeed the signal PTS. Advantageously, during step b), said digital clock signal is at a frequency of kxc3x9727 MHz where k is a non-zero integer or fraction.
The invention also provides a system for measuring the quality of signals in a digital television network having a plurality of transmission installations, the system being characterized in that it includes a plurality of measurement equipments associated with at least some of the transmission installations, and each of which comprises:
an identifier device receiving the data stream as input and outputting a reference time signal selected from the data stream;
a clock generator whose phase is locked on the basis of said reference time signal and which outputs a multibit digital clock signal operating at a given frequency, and whose count is deduced unambiguously form the count of a time base generated while encoding the digital television signals;
a device for labelling quality measurements using at least the meaningful bits of the multibit digital clock signal; and
a device for transmitting the labelled quality measurements: and in that it includes a receiver for collecting the labelled measurement data transmitted by at least one of said transmission devices.
Preferably, the count of the multibit digital clock signal is equal to the count of said time base.
The labelling procedure of the invention makes it possible to compare measurements performed on the same data at various points in the network, without data recognition being affected by propagation delays due to transmission and/or encoding and/or decoding.